1. Field of the Invention
The subject invention pertains to the art of signal detection and more specifically to a quantization detection system wherein the signal level in a detection cell of interest is compared with the signal levels in detection cells on either side thereof, a signal being declared in the detection cell of interest when the signal level therein exceeds the signal level in a predetermined number of detection cells with which it is compared.
2. Description of the Prior Art
A number of detection techniques for radar, sonar, and other detection systems have been proposed in which the detection threshold is automatically adjusted in accordance with variations in background noise and interference. These systems include rank sum and rank quantization detection. A rank sum detector passes a video signal through a tapped delay line whereon the signals at each tap represent the signals within a corresponding detection cell and on which the detection cell of interest is represented by the central tap. The rank of the central tap is determined by comparing the signal amplitude thereat to the signal amplitude at each of the other taps, this rank being the number of detection cells wherein the amplitude is less than the amplitude in the detection cell represented by the central tap. When the sum of the ranks obtained over several range sweeps exceeds a specified threshold, a signal detection for the central tap is declared.
The rank quantization detector like the rank sum detector compares the signal level at the center tap of a tapped delay line to the signal levels at all the other taps. Instead of obtaining a rank sum after a number of transmitted pulses and comparing the rank sum to a given threshold, the rank quantization detector compares the rank obtained after each transmitted pulse to a specified threshold, sums the number of ranks exceeding this threshold after a number of transmitted pulses, and compares the sum to a second threshold to determine a signal detection.
These prior art systems provide approximately the same level of performance and exhibit substantially the same deficiencies. One such deficiency involves the masking of a signal in the detection cell of interest by an intefering signal in a detection cell represented by one of the taps on the tapped delay line. For example, if N taps are available on the delay line and a rank threshold equal to N is established, a signal appearing at any one of the taps on the delay line that is larger than the signal at the tap representative of the detection cell of interest will effectively prevent detection of the desired signal. In such a situation, the rank threshold may be reduced, with a concomitant increase in the false alarm rate, or the signals entering the tapped delay line may be pre-processed such that any interfering signal effects only one tap, so that a rank threshold of N-1 can be used. The latter approach introduces significant signal correlation between the signals appearing at the various taps along the delay lines, thus destroying the distribution free property of the rank detector.
The present invention provides a detection system which maintains a low false alarm rate and a distribution free property in the presence of relatively large interfering signals.